| 1. |
- Ambatipudi, Radhika, 1982-
(author)
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High Frequency (MHz) Planar Transformers for Next Generation Switch Mode Power Supplies
- 2013
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Doctoral thesis (other academic/artistic)abstract
- Increasing the power density of power electronic converters while reducing or maintaining the same cost, offers a higher potential to meet the current trend inrelation to various power electronic applications. High power density converters can be achieved by increasing the switching frequency, due to which the bulkiest parts, such as transformer, inductors and the capacitor's size in the convertercircuit can be drastically reduced. In this regard, highly integrated planar magnetics are considered as an effective approach compared to the conventional wire wound transformers in modern switch mode power supplies (SMPS). However, as the operating frequency of the transformers increase from several hundred kHz to MHz, numerous problems arise such as skin and proximity effects due to the induced eddy currents in the windings, leakage inductance and unbalanced magnetic flux distribution. In addition to this, the core losses whichare functional dependent on frequency gets elevated as the operating frequency increases. Therefore, this thesis provides an insight towards the problems related to the high frequency magnetics and proposes a solution with regards to different aspects in relation to designing high power density, energy efficient transformers.The first part of the thesis concentrates on the investigation of high power density and highly energy efficient coreless printed circuit board (PCB) step-down transformers useful for stringent height DC-DC converter applications, where the core losses are being completely eliminated. These transformers also maintain the advantages offered by existing core based transformers such as, high coupling coefficient, sufficient input impedance, high energy efficiency and wide frequencyband width with the assistance of a resonant technique. In this regard, several coreless PCB step down transformers of different turn’s ratio for power transfer applications have been designed and evaluated. The designed multilayered coreless PCB transformers for telecom and PoE applications of 8,15 and 30W show that the volume reduction of approximately 40 - 90% is possible when compared to its existing core based counterparts while maintaining the energy efficiency of the transformers in the range of 90 - 97%. The estimation of EMI emissions from the designed transformers for the given power transfer application proves that the amount of radiated EMI from a multilayered transformer is lessthan that of the two layered transformer because of the decreased radius for thesame amount of inductance.The design guidelines for the multilayered coreless PCB step-down transformer for the given power transfer application has been proposed. The designed transformer of 10mm radius has been characterized up to the power level of 50Wand possesses a record power density of 107W/cm3 with a peak energy efficiency of 96%. In addition to this, the design guidelines of the signal transformer fordriving the high side MOSFET in double ended converter topologies have been proposed. The measured power consumption of the high side gate drive circuitvitogether with the designed signal transformer is 0.37W. Both these signal andpower transformers have been successfully implemented in a resonant converter topology in the switching frequency range of 2.4 – 2.75MHz for the maximum load power of 34.5W resulting in the peak energy efficiency of converter as 86.5%.This thesis also investigates the indirect effect of the dielectric laminate on the magnetic field intensity and current density distribution in the planar power transformers with the assistance of finite element analysis (FEA). The significanceof the high frequency dielectric laminate compared to FR-4 laminate in terms of energy efficiency of planar power transformers in MHz frequency region is also explored.The investigations were also conducted on different winding strategies such as conventional solid winding and the parallel winding strategies, which play an important role in the design and development of a high frequency transformer and suggested a better choice in the case of transformers operating in the MHz frequency region.In the second part of the thesis, a novel planar power transformer with hybrid core structure has been designed and evaluated in the MHz frequency region. The design guidelines of the energy efficient high frequency planar power transformerfor the given power transfer application have been proposed. The designed corebased planar transformer has been characterized up to the power level of 50W and possess a power density of 47W/cm3 with maximum energy efficiency of 97%. This transformer has been evaluated successfully in the resonant converter topology within the switching frequency range of 3 – 4.5MHz. The peak energy efficiency ofthe converter is reported to be 92% and the converter has been tested for the maximum power level of 45W, which is suitable for consumer applications such as laptop adapters. In addition to this, a record power density transformer has been designed with a custom made pot core and has been characterized in thefrequency range of 1 - 10MHz. The power density of this custom core transformer operating at 6.78MHz frequency is 67W/cm3 and with the peak energy efficiency of 98%.In conclusion, the research in this dissertation proposed a solution for obtaining high power density converters by designing the highly integrated, high frequency(1 - 10MHz) coreless and core based planar magnetics with energy efficiencies inthe range of 92 - 97%. This solution together with the latest semiconductor GaN/SiC switching devices provides an excellent choice to meet the requirements of the next generation ultra flat low profile switch mode power supplies (SMPS).
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| 2. |
- Ashraf, Shakeel, 1984-, et al.
(author)
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Design of a multilayered absorber structure based on SU-8 epoxy for broad and efficient absorption inMid-IR sensitive thermal detectors
- 2014
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In: Proceedings. ; , s. 938-941
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Conference paper (peer-reviewed)abstract
- This paper reports on design, simulation and fabrication of a multilayered interferometric absorption structure with a broad absorption in the mid-infrared band. This region is used for IR based CH4 and CO2 detection. The structure consists of five layers of different thickness. The structure consists of one mirror layer of aluminium, two SU-8 epoxy layers and two thin titanium layers. This structure has been fabricated on a silicon substrate and verified for its absorption properties through Fourier transform infrared spectroscopy. The fabricated structure has been compared with simulations are performed using transfer matrix theory. The structure shows more than 90% absorption in the wavelength range of 3.20μm - 5.35μm for simulations and 3.13μm - 5.47μm for FT-IR measurements. The transmission and reflection of SU-8 epoxy was measured using FT-IR (that), resulting in a calculated absorption between 10 - 20% in the area of interest (3μm - 6μm). The use of SU-8 epoxy as dielectric medium, allows for direct integration of the structure into the membrane of a SU-8 membrane based thermopile. The integration results in minimum increase of the thermal capacitance and conductance, which results in maximum detector sensitivity and minimum time constant.
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| 3. |
- Ashraf, Shakeel, 1984-, et al.
(author)
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Integration of an interferometric IR absorber into an epoxy membrane based CO2 detector
- 2014
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In: Journal of Instrumentation. - 1748-0221. ; 9:5, s. Art. no. C05035-
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Journal article (peer-reviewed)abstract
- Measurements of carbon dioxide levels in the environment are commonly performedby using non-dispersive infrared technology (NDIR). Thermopile detectors are often used in NDIRsystems because of their non-cooling advantages. The infrared absorber has a major influence onthe detector responsivity. In this paper, the fabrication of a SU-8 epoxy membrane based Al/Bithermopile detector and the integration of an interferometric infrared absorber structure of wavelength around 4 µ m into the detector is reported. The membrane of thermopile detector has beenutilized as a dielectric medium in an interferometric absorption structure. By doing so, a reduction in both thermal conductance and capacitance is achieved. In the fabrication of the thermopile,metal evaporation and lift off process had been used for the deposition of serially interconnectedAl/Bi thermocouples. Serial resistance of fabricated thermopile was measured as 220 kΩ. Theresponse of fabricated thermopile detector was measured using a visible to infrared source of radiation flux 3.23 mW mm−2. The radiation incident on the detector was limited using a band passfilter of wavelength 4.26 µ m in front of the detector. A responsivity of 27.86 V mm2W−1at roomtemperature was achieved using this setup. The fabricated detector was compared to a referencedetector with a broad band absorber. From the comparison it was concluded that the integratedinterferometric absorber is functioning correctly.
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| 4. |
- Esebamen, Omeime, et al.
(author)
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Surface State Effects on N+P Doped Electron Detector
- 2011
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In: Journal of Instrumentation. - 1748-0221. ; 6:12, s. Art. no. C12019-
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Journal article (peer-reviewed)abstract
- Surface states and interface recombination velocity that exist between detector interfaces have always been known to affect the performance of a detector. This article describes how the detector performance varies when the doping profile is altered. When irradiated with electrons, the results show that while changes in the doping profile have an effect of the detector responsivity with respect to the interface recombination velocityVs, there is no visible effect with respect tofixed oxide chargeQfotherwise known as interface fixed charge density.
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| 5. |
- Esebamen, Omeime Xerviar, et al.
(author)
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A Different Approach of Determining the Responsivity of n+p Detectors Using Scanning Electron Microscopy
- 2012
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In: Journal of semiconductors. - : IOP Publishing. - 1674-4926. ; 33:7, s. 074002-
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Journal article (peer-reviewed)abstract
- This paper explores an alternative to the standard method of studying the responsivities (the input—output gain) and other behaviours of detectors at low electron energy. The research does not aim to compare the results of differently doped n+p detectors; its purpose is to provide an alternative characterization method (using scanning electron microscopy) to those used in previous studies on the responsivity of n+p doped detectors as a function of the electron radiation energy and other interface parameters.
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| 6. |
- Esebamen, Omeime Xerviar
(author)
-
Characterization of interface states & radiation damage effects in duo-lateral PSDs : Using SEM microscopy and UV beam profiling techniques
- 2014
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Doctoral thesis (other academic/artistic)abstract
- There has been an increase in the use of duo‐lateral position sensitive detectors inpractically every radiation and beam detection application. These devices unlike other light detection system utilize the effect of the lateral division of the generated photocurrent to measure the position of the integral focus of an incoming lightsignal. The performance of a PSD is impaired or strengthened by a number of events caused by parameters such as interface states and recombination introduced during the fabrication of the detector and/or its absorption of ionizing particles. This thesis show the results from the successful implementation of alternative characterization methods of these effects and parameters using scanning electronmicroscopy and UV beam profiling techniques on duo‐lateral position sensitive detectors (LPSDs). To help create the groundwork for the research content of this thesis, different technical reviews of previous studies on interface states, surface recombination velocity and radiation damage due to continuous absorption of ionizing irradiation on detectors are investigated. The thesis also examines published theoretical and measurement techniques used to characterize these surface/interface phenomena. The PSDs used in this research were developed using silicon technology and the various methodologies put into the fabrication of the detectors (n+p and p+n structures) were fashioned after the simulated models. The various steps associated with the clean room fabrication and the prior simulation steps are highlighted in the content of the thesis. Also discussed are the measurement techniques used incharacterizing the fixed oxide charge, surface recombination and the position deviation error of the LPSDs in a high vacuum environment of a scanning electron microscope SEM chamber. Using this method, the effects of interface states and surface recombination velocity on the responsivity of differently doped LPSDs were investigated. By lithographically patterning grid‐like structures used as scaleon n+p doped LPSD and using sweeping electrons from the SEM microscope, a very high linearity over the two‐dimensions of the LPSD total active area was observed. An improved responsivity for low energetic electrons was also achievedby the introduced n+p structure. The lithographically patterned grids helped eliminate further external measurement errors and uncertainties from the use of other typical movable measurement devices such as actuators and two dimensional adjusters which would normally be difficult to install in a remote vacuum chamber. In a similar vein, field plate and field rings were patterned around an array ofthe PSDs used as pixel detector(s). By studying the interpixel resistance and breakdown characteristics, the most effective structural arrangement of the field plate and field rings used to curb induced inversion channel between the n+ doped regions of the pixel‐detector is observed. By using UV beam profiling after the irradiation of UV (193 nm or 253 nm) beam on n+p and p+n doped PSDs, the degree of radiation damage was also investigated. The results obtained help to illustrate how prolonged UV radiation can impact on the linearity and the position deviation/error of UV detectors. The results in this thesis are most relevant in spectroscopic and microscopic applications where low energy electrons and medium UV (MUV) radiation are used.
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| 7. |
- Esebamen, Omeime Xerviar, et al.
(author)
-
Comparative Study of UV Radiation Hardness of n+p and p+n Duo-Lateral Position Sensitive Detectors
- 2014
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In: European Physical Journal. - : EDP Sciences. - 1286-0042 .- 1286-0050. ; 68:2, s. Art. no. 21301-
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Journal article (peer-reviewed)abstract
- We report experimental results on the degree of radiation damage in two duo-lateral position sensitive detectors (LPSDs) exposed to 193 nm and253 nm ultraviolet (UV) beam. One of the detectors was an in-house fabricated n(+) p LPSD and the other was a commercially available p(+) n LPSD. We report that at both wavelengths, the degradation damage from the UV photons absorption caused a much more significant deterioration in responsivity in the p(+) n LPSD than in the n(+) p LPSD. By employing a simple method, we were able to visualize the radiation damage on the active area of the LPSDs using 3-dimensional graphs. We were also able to characterize the impact of radiation damage on the linearity and position error of the detectors.
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| 8. |
- Esebamen, Omeime Xerviar, et al.
(author)
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Gridded Duo-lateral Position Sensitivity Detector with High Linearity to Low Energetic Electrons in Vacuum Environment
- 2014
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In: IET Optoelectronics. - : Institution of Engineering and Technology (IET). - 1751-8768 .- 1751-8776. ; 8:6, s. 217-225
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Journal article (peer-reviewed)abstract
- Characterizing a position sensitive detector in a vacuum environment without beam position monitoring devices can bechallenging and expensive. With this in mind, we have designed and fabricated a duo-lateral position sensitive detector (PSD) incorporatedwith simple and inexpensive surface features. It was evaluated using scanning electron microscopy. To assist in pinpointing precise positioningas well as acting as path guide during the sweeping of electrons, multiple grids were lithographically patterned on the top layer of the duolateralPSD. By sweeping electrons along two axes of the detector, the position detection error of both axes was determined from the signalsrecorded using a transimpedance amplification circuit. We were able to characterize the linearity over the x- and y-axis of the PSD and theresults show a very high linearity over two-dimensions of the PSD’s active area and that accurate beam monitoring for spectroscopic measurement without additional beam position monitoring devices is possible.
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| 9. |
- Esebamen, Omeime X., et al.
(author)
-
High resolution, low energy electron detector
- 2011
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In: Journal of Instrumentation. - 1748-0221. ; 6:1, s. Art. no. P01001-
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Journal article (peer-reviewed)abstract
- Electron detection at low energy range for scanning electron microscope (SEM), electron capture detector and electron probe micro-analysis (EPMA) applications, require detectors with high sensitivity and accuracy for low energy range. Such detectors must therefore have a thin entrance window and low recombination at the Si-SiO2 interface. An electron detector with 100 photons to electron-hole pair production rate having a 10 nm SiO2 passivating layer reveals a responsivity of approximately 0.25 A/W when irradiated. Simulations results showing the responsivity of electron interaction between detectors of varied interface fixed oxide charge density Qf show that there is an appreciable difference with the responsivity of a p +n detector and that of an n+p. The simulation results also show the significance of the effect of the minority carriers transport velocity Sn,p on the responsivity of the detector. © 2011 IOP Publishing Ltd and SISSA.
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| 10. |
- Esebamen, Omeime Xerviar
(author)
-
Simulation, Measurement and Analysis of the Response of Electron- and Position Sensitive Detector
- 2012
-
Licentiate thesis (other academic/artistic)abstract
- Different methods exist in relation to probing and investigating thephysical and structural composition of materials especially detectors whoseusage have become an integral part of radiation detection. The use of thescanning electron microscopy is just one of such exploratory methods. Thistechnique uses a focused beam of high-energy electrons to generate a varietyof signals at the surface of the device under investigationThis thesis presents the results derived from signals from electron beamsampleinteractions, revealing information about the different cleanroomfabricated electron detectors used. This information includes the detector’sexternal morphology and texture, surface recombination, fixed oxide chargeand the behavioral characteristic in the form of its position detection accuracyand linearity.An electron detector with a high ionization factor and which has a 10nmSilicon Oxide passivating layer was fabricated. Results from using the scanningelectron microscopy showed that its maximum responsivity wasapproximately 0.25 A/W from a possible 0.27 A/W. In conjunction withsimulations, results also showed the significance of the effect of the minoritycarrier's surface recombination velocity on the responsivity of the detectors.In addition, measurements were conducted to ascertain the performancevariance of these electron detectors with respect to their surfacerecombination velocity and fixed oxide charge when the doping profile isaltered.By incorporating special features on a fabricated duo-lateral positionsensitive detector (PSD), a position sensing resolution of the PSD using theelectron microscopic method was also evaluated. The evaluation showed avery high linearity over two-dimensions for 77% of the PSD’s active area.The results in this thesis offer a significant improvement in electrondetectors for applications such as gas chromatography detection of traceamounts of chemical compounds in a sample as well as applications involvingposition sensitive detection.
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| 11. |
- Fröjdh, Anna, et al.
(author)
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An alpha particle detector for measuring radon levels
- 2010
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In: Proceedings of Nuclear Science Symposium Conference Record (NSS/MIC), 2010. - : IEEE conference proceedings. - 9781424491063 ; , s. 460-461
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Conference paper (peer-reviewed)abstract
- An alpha particle detector for measuring radon levels through measurement of radon progeny concentration has been developed. The detector is a silicon diode optimized for these measurements. Different alternating and non-alternating guard ring structures and different doping profiles have been investigated.
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| 12. |
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| 13. |
- Fröjdh, Anna, 1986-, et al.
(author)
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Processing and characterization of a MEDIPIX2-compatible silicon sensor with 220 mu m pixel size
- 2011
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In: Nuclear Instruments and Methods in Physics Research Section A. - : Elsevier BV. - 0168-9002 .- 1872-9576. ; 633:Suppl 1, s. S78-S80
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Journal article (peer-reviewed)abstract
- Pixellated silicon detectors with a pixel size of 220 um have been fabricated at Mid Sweden University. The detectors will be bonded to the MEDIPIX2 [1] readout chip. The purpose is to investigate the performance of an energy sensitive X-ray imaging sensor with reduced charge sharing.The detectors were fabricated on high purity silicon with a wafer thickness of 500 um and a resistivity of more than 15 kohmcm. One reason for the choice of material was to get experience for future work with very thick detectors requiring ultra high resistivity in order to be depleted. During the initial work in this project some issued were found concerning inter pixel resistance and the efficiency of the guard rings. This led to a study of existing papers on the subject [2,3,4,5] and to extensive simulations of the electric field and the charge transport in different parts of the device.A modified process has been developed using alternating p+ and n+ guard rings and an outer n+ doping. The results of the simulations and the process will be described as well as an outline for a process for fabrication of very thick detectors with limited guard ring extension.References[1] - X. Llopart, M. Campbell, R. Dinapoli, D. San Segundo and E. Pernigotti, IEEE Trans. Nucl. Sci., vol. 49, 2279-2283, October 2002.[2] – L. Evensen, A. Hanneborg, B Sundby Avset, M. Mese, Nuclear Instruments and Methods in Physics Research A 337 (1993) 44 – 52[3] – T. Pavalainen, T. Tuuva, K. Leinonen, Nuclear Instruments and Methods in Physics Research A 573 (2007) 277 – 279[4] – Z. Li, W. Huang, L. J. Zhao, IEEE Trans. Nucl. Sci., vol. 47, No. 3. 729 – 735 , June 2000.[5] – D. Han, C. Wang, G. Wang, S. Du, L. Shen, X. Tian, X. Zhang, IEEE Transactions on Electron devices, Vol. 50, No. 2, 537 – 540, February 2003
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| 14. |
- Fröjdh, Christer, et al.
(author)
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Spectral resolution in pixel detectors with single photon processing
- 2013
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In: Proceedings SPIE Optical Engineering + Applications, 2013. - : SPIE - International Society for Optical Engineering. - 9780819497024
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Conference paper (other academic/artistic)abstract
- Pixel detectors based on photon counting or single photon processing readout are becoming popular for spectral X-ray imaging. The detector is based on deep submicron electronics with functions to determine the energy of each individual photon in every pixel. The system is virtually noiseless when it comes to the number of the detected photons. However noise and variations in system parameters affect the determination of the photon energy. Several factors affect the energy resolution in the system. In the readout electronics the most important factors are the threshold dispersion, the gain variation and the electronic noise. In the sensor contributions come from charge sharing, variations in the charge collection efficiency, leakage current and the statistical nature of the charge generation, as described by the Fano factor. The MEDIPIX technology offers a powerful tool for investigating these effects since energy spectra can be captured in each pixel. In addition the TIMEPIX chip, when operated in Time over Threshold mode, offers an opportunity to analyze individual photon interactions, thus addressing charge sharing and fluorescence. Effects of charge sharing and the properties of charge summing can be investigated using MEDIPIX3RX. Experiments are performed using both Si and CdTe detectors. In this paper we discuss the various contributions to the spectral noise and how they affect detector response. The statements are supported with experimental data from MEDIPIX-type detectors.
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| 15. |
- Fröjdh, Erik, et al.
(author)
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Depth of interaction and bias voltage dependence of the spectral response in a pixellated CdTe detector operating in time-over-threshold mode subjected to monochromatic X-rays
- 2012
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In: Journal of Instrumentation. - 1748-0221. ; 7:3, s. Art. no. C03002-
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Journal article (peer-reviewed)abstract
- High stopping power is one of the most important figures of merit for X-ray detectors. CdTe is a promising material but suffers from: material defects, non-ideal charge transport and long range X-ray fluorescence. Those factors reduce the image quality and deteriorate spectral information. In this project we used a monochromatic pencil beam collimated through a 20Όm pinhole to measure the detector spectral response in dependance on the depth of interaction. The sensor was a 1mm thick CdTe detector with a pixel pitch of 110Όm, bump bonded to a Timepix readout chip operating in Time-Over-Threshold mode. The measurements were carried out at the Extreme Conditions beamline I15 of the Diamond Light Source. The beam was entering the sensor at an angle of ∌20 degrees to the surface and then passed through ∌25 pixels before leaving through the bottom of the sensor. The photon energy was tuned to 77keV giving a variation in the beam intensity of about three orders of magnitude along the beam path. Spectra in Time-over-Threshold (ToT) mode were recorded showing each individual interaction. The bias voltage was varied between -30V and -300V to investigate how the electric field affected the spectral information. For this setup it is worth noticing the large impact of fluorescence. At -300V the photo peak and escape peak are of similar height. For high bias voltages the spectra remains clear throughout the whole depth but for lower voltages as -50V, only the bottom part of the sensor carries spectral information. This is an effect of the low hole mobility and the longer range the electrons have to travel in a low field. © 2012 IOP Publishing Ltd and Sissa Medialab srl.
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| 16. |
- Fröjdh, Erik, 1984-, et al.
(author)
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Mapping the x-ray response of a CdTe sensor with small pixels using an x-ray microbeam and a single photon processing readout chip
- 2011
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In: Proceedings of SPIE - The International Society for Optical Engineering. - : SPIE - International Society for Optical Engineering. - 9780819487520 ; , s. Art. no. 814208-
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Conference paper (peer-reviewed)abstract
- CdTe is a promising material for X-ray imaging since it has high stopping power for X-rays. However defects in the material, non ideal charge transport and long range X-ray fluorescence deteriorates the image quality. We have investigated the response of a CdTe sensor with very small pixels using an X-ray microbeam entering the sensor at a small incident angle. Effects of defects as well as depth of interaction can be measured by this method. Both electron and hole collection mode has been tested. The results show distorted electrical field around defects in the material and also shows the small pixel effect. It is also shown that charge summing can be used to get correct spectral information.
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| 17. |
- Fröjdh, Erik, 1984-, et al.
(author)
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Probing Defects in a Small Pixellated CdTe Sensor Using an Inclined Mono Energetic X-Ray Micro Beam
- 2012
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In: 2012 IEEE NUCLEAR SCIENCE SYMPOSIUM AND MEDICAL IMAGING CONFERENCE RECORD (NSS/MIC). - : IEEE conference proceedings. - 9781467320306 - 9781467320283 ; , s. 4233-4236
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Conference paper (other academic/artistic)abstract
- High quantum efficiency is important in X-ray imaging applications. This means using high-Z sensor materials. Unfortunately many of these materials suffer from defects that cause non-ideal charge transport. In order to increase the understanding of these defects, we have mapped the 3D response of a number of defects in two 1 mm thick CdTe sensors with different pixel sizes (55 mu m and 110 mu m) using a monoenergetic microbeam at 79 keV. The sensors were bump bonded to Timepix read out chips. Data was collected in photon counting as well as time-over-threshold mode. The time-over-threshold mode is a very powerful tool to investigate charge transport properties and fluorescence in pixellated detectors since the signal from the charge that each photon deposits in each pixel can be analyzed. Results show distorted electrical field around the defects and indications of excess leakage current and large differences in behavior between electron collection and hole collection mode. The experiments were carried out in the Extreme Conditions Beamline I15 at Diamond Light Source.
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| 18. |
- Fröjdh, Erik, et al.
(author)
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Probing Defects in a Small Pixellated CdTe Sensor Using an Inclined Mono Energetic X-Ray Micro Beam
- 2013
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In: IEEE Transactions on Nuclear Science. - 0018-9499 .- 1558-1578. ; 60:4, s. 2864-2869
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Journal article (peer-reviewed)abstract
- High quantum efficiency is important in X-ray imaging applications. This means using high-Z sensor materials. Unfortunately many of these materials suffer from defects that cause non-ideal charge transport. In order to increase the understanding of these defects, we have mapped the 3D response of a number of defects in two 1 mm thick CdTe sensors with different pixel sizes (55 mu m and 110 mu m) using a monoenergetic microbeam at 79 keV. The sensors were bump bonded to Timepix read out chips. Data was collected in photon counting as well as time-over-thresholdmode. The time-over-thresholdmode is a very powerful tool to investigate charge transport properties and fluorescence in pixellated detectors since the signal from the charge that each photon deposits in each pixel can be analyzed. Results show distorted electrical field around the defects, indications of excess leakage current and large differences in behavior between electron collection and hole collection mode. The experiments were carried out on the Extreme Conditions Beamline I15 at Diamond Light Source.
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| 19. |
- Fröjdh, Erik, 1984-, et al.
(author)
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Spectral Response in a Pixellated X-ray Imaging CdTe Detector with Single Photon Processing Readout
- 2010
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In: Proceeddings of 2010 Nuclear Science Symposium and Medical Imaging Conference. - : IEEE conference proceedings. - 9781424491063 ; , s. 1079-1080
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Conference paper (other academic/artistic)abstract
- The image forming process in a CdTe detector is both a function of the X-ray interaction in the material, including scattering and fluorescence, and the charge transport in the material [2]-[4]. The response of individual photons has been investigated using two pixellated CdTe image detectors with pixel pitches of 55 mu m and 110 mu m. The detectors were bonded to TIMEPIX [5] readout chips and operated in time over threshold mode (ToT). We have illuminated the sensors with monoengertic photons generated by X-ray fluorescence in metal sheets and with gamma photons from Am-241 and Cs-137. Results shows a large degradation in energy resolution caused by charge sharing and fluorescence. By summing pixels together we can correct for the charge sharing and some, but not all fluorescence.
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| 20. |
- Fröjdh, Erik, et al.
(author)
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X-ray absorption and charge transport in a pixellated CdTe detector with single photon processing readout
- 2011
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In: Journal of Instrumentation. - 1748-0221. ; 6:2, s. Art. no. P02012-
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Journal article (peer-reviewed)abstract
- The image forming process in a CdTe detector is both a function of the X-ray interaction in the material, including scattering and fluorescence, and the charge transport in the material [2-4]. The response to individual photons has been investigated using a CdTe detector with a pixel size of 110 m m, bonded to a TIMEPIX [5] readout chip operating in time over threshold mode. The device has been illuminated with mono-energetic photons generated by fluorescence in different metals and by gamma emission from Am-241 and Cs-137. Each interaction will result in charge distributed in a cluster of pixels where the total charge in the cluster should sum up to the initial photon energy. By looking at the individual clusters the response from shared photons as well as fluorescence photons can be identified and separated. By using energies below and above the K-edges of Cd and Te the contribution from fluorescence can be further isolated. The response is analyzed to investigate the effects of both charge diffusion and fluorescence on the spectral response in the detector.
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| 21. |
- Kok, A., et al.
(author)
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Silicon sensors with pyramidal structures for neutron imaging
- 2014
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In: Journal of Instrumentation. - 1748-0221. ; 9, s. C04011-
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Journal article (peer-reviewed)abstract
- Neutron detection is a valuable tool in nuclear science research, homeland security, quality assurance in nuclear plants and medical applications. Recent developments and near future instrumentations in neutron imaging have a need for sensors with high spatial resolution, dynamic range, sensitivity and background discrimination. Silicon based neutron detectors can potentially fulfil these requirements. In this work, pad and pixel detectors with pyramidal micro-structures have been successfully fabricated that should have an improved detection efficiency when compared to conventional planar devices. Titanium di-boride (TiB2) and lithium fluoride (LiF) were deposited as the neutron converters. Excellent electrical performances were measured on both simple pad and pixel detectors. A selection of pad detectors was examined by alpha spectroscopy. Measurement with thermal neutrons from a 241Am-Be source shows an improvement in relative efficiency of up to 38% when compared to conventional planar devices.
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| 22. |
- Krapohl, David, et al.
(author)
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Simulation and measurement of short infrared pulses on silicon position sensitive device
- 2011
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In: Journal of Instrumentation. - 1748-0221. ; 6:C01036
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Journal article (peer-reviewed)abstract
- Lateral position sensitive devices (PSD) are important for triangulation, alignment and surface measurements as well as for angle measurements. Large PSDs show a delay on rising and falling edges when irradiated with near infra-red light [1]. This delay is also dependent on the spot position relative to the electrodes. It is however desirable in most applications to have a fast response. We investigated the responsiveness of a Sitek PSD in a mixed mode simulation of a two dimensional full sized detector. For simulation and measurement purposes focused light pulses with awavelength of 850 nm, duration of 1 µs and spot size of 280 µm were used. The cause for the slopes of rise and fall time is due to time constants of the device capacitance as well as the photo- generation mechanism itself [1]. To support the simulated results, we conducted measurements of rise and fall times on a physical device. Additionally, we quantified the homogeneity of the device by repositioning a spot of light from a pulsed ir-laser diode on the surface area.
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| 23. |
- Krapohl, David, et al.
(author)
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Simulation of a silicon neutron detector coated with TiB 2 absorber
- 2012
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In: Journal of Instrumentation. - 1748-0221. ; 7:1, s. Art. no. C01096-
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Journal article (peer-reviewed)abstract
- Neutron radiation cannot be directly detected in semiconductor detectors and therefore needs converter layers. Planar clean-room processing can be used in the manufacturing process of semiconductor detectors with metal layers to produce a cost-effective device. We used the Geant4 Monte-Carlo toolkit to simulate the performance of a semiconductor neutron detector. A silicon photo-diode was coated with vapour deposited titanium, aluminium thin films and a titaniumdiboride (TiB 2) neutron absorber layer. The neutron capture reaction 10B(n, alpha)7Li is taken advantage of to create charged particles that can be counted. Boron-10 has a natural abundance of about SI 19.8%. The emitted alpha particles are absorbed in the underlying silicon detector. We varied the thickness of the converter layer and ran the simulation with a thermal neutron source in order to find the best efficiency of the TiB 2 converter layer and optimize the clean room process. © 2012 IOP Publishing Ltd and SISSA.
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| 24. |
- Krapohl, David, 1980-, et al.
(author)
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Simulation of the Spectral Response of a Pixellated X-Ray Imaging Detector in Single Photon Processing Mode
- 2010
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In: 2010 Nuclear Science Symposium and Medical Imaging Conference. - : IEEE conference proceedings.
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Conference paper (peer-reviewed)abstract
- X-ray imaging with spectral resolution, “Color X-ray imaging” is a new imaging technology that is currently attracting a lot of attention. It has however been observed that the quality of spectral response is degraded as the pixel size is reduced. This is an effect of charge sharing where the signal from a photon absorbed close to the border between two pixels is shared between pixels. This effect is caused by both diffusion during the charge transport and X-ray fluorescence in heavy detector materials. In order to understand the behavior of pixellated detectors with heavy detector materials operating in single photon processing mode, we have simulated the X-ray interaction with the sensor and the transport of the charge to the readout electrode using a Monte Carlo model for the X-ray interaction and a drift diffusion model for the charge transport. By combining these models, both signal and noise properties of the detector can be simulated.
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| 25. |
- Norlin, Börje, 1967-, et al.
(author)
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Spectroscopic X-Ray Imaging Using a Pixelated Detector with Single Photon Processing Readout
- 2010
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In: Proceedings of 2010 IEEE NUCLEAR SCIENCE SYMPOSIUM CONFERENCE. - : IEEE conference proceedings. - 9781424491063 ; , s. 1074-1078
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Conference paper (peer-reviewed)abstract
- Color informatics to analyze the object content in Xray images is an emerging technology. Identification of different elements for applications such as medical contrast agent imaging is possible using energy resolving X-ray imaging sensors. In this work RGB representations of transmission images of ground elements achieved with the MEDIPIX system are exemplified.
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| 26. |
- Schübel, Armin, et al.
(author)
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A Geant4 based framework for pixel detector simulation
- 2014
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In: Journal of Instrumentation. - 1748-0221. ; 9:12
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Journal article (peer-reviewed)abstract
- The output from a hybrid pixel detector depends on the interaction of the radiation with the sensor material, the transport of the resulting charge in the sensor, the pulse processing in the readout circuit and processing of the resulting signal. In order to understand the full behaviour of the device and to predict the performance of future devices it is important to have a framework that can simulate the entire process in the detector system.Geant4 is a Monte Carlo based toolkit for simulation of particle interaction with matter which is developed and actively used for CERN experiments and detector development [1]. By extending the Monte Carlo code in Geant4 with a charge carrier transport model of the sensor material and basic amplifier functionality as well as read out logic, a simulation of the complete detector system is possible.The MEDIPIX is a state of the art hybrid pixel detector that allows bonding of a wide range of sensor materials [2,3]. Simulation models have been developed and tested for different chips from the MEDIPIX family. The simulation is defined using configuration files to set the geometry, sensor material properties, number of pixels, pixel pitch and chip properties. Source properties as well as filters and objects in the beam can be added for different experimental set-ups. The interaction of radiation with the sensor is taken into account in the transport of the charge carriers in the sensor material and a current induced in the pixel electrode that triggers an amplifier response. Simulation results have been verified with X-ray fluorescence and radioactive sources using MEDIPIX family chips. In this paper we present the developed simulation framework and first results.
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| 27. |
- Slavicek, Tomas, et al.
(author)
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A thermal neutron detector based on planar silicon sensor with TiB 2 coating
- 2012
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In: Journal of Instrumentation. - 1748-0221. ; 7:1, s. Art. no. C01053-
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Journal article (peer-reviewed)abstract
- Neutron radiation as a non-ionizing radiation is particularly difficult to detect; therefore a conversion material is required. The conversion material converts neutrons into secondary charged particles in order for them to be detected in a silicon detector. The use of titanium diboride (TiB 2) as the conversion material deposited by an electron beam-physical vapour deposition (EB-PVD) as a part of a front-side contact of a planar silicon detector is presented. The effect of different front-side contact material compositions is discussed. The detectors behaviour was examined using alpha particles and thermal neutrons from an 241Am-Be source. Simultaneously, a Geant4 simulation was so as executed to evaluate the conversion layer functionality and to discover the conversion material thickness for the best neutron detection efficiency. © 2012 IOP Publishing Ltd and SISSA.
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| 28. |
- Thungström, Göran, et al.
(author)
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Fabrication, characterization and simulation of channel stop for n in p-substrate silicon pixel detectors
- 2014
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In: Journal of Instrumentation. - 1748-0221. ; 9, s. C07013-
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Journal article (peer-reviewed)abstract
- Silicon detectors made on p-substrates are expected to have a better radiation hardness as compared to detectors made on n-substrates. However, the fixed positive oxide charges induce an inversion layer of electrons in the substrate, which connects the pixels. The common means of solving this problem is by using a p-spray, individual p-stops or a combination of the two. Here, we investigate the use of field plates to suppress the fixed positive charges and to prevent the formation of an inversion layer. The fabricated detector shows a high breakdown voltage and low interpixel leakage current for a structure using biased field plates with a width of 20 m m. By using a spice model for simulation of the preamplifier, a cross talk of about 1.6% is achieved with this detector structure. The cross talk is caused by capacitive and resistive coupling between the pixels.
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| 29. |
- Thungström, Göran, et al.
(author)
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Measurement of the sensitive profile in a solid state silicon detector, irradiated by X-rays
- 2013
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In: Journal of Instrumentation. - : IOP. - 1748-0221. ; 8
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Journal article (peer-reviewed)abstract
- A newly constructed solid state silicon dose profile detector is characterized concerning its sensitive profile. The use of the MEDIPIX2 sensor system displays an excellent method to align an image of an X-ray slit to a sample under test. The scanning from front to reverse side of the detector, show a decrease in sensitivity of 20%, which indicates a minority charge carrier lifetime of 0.18 ms and a diffusion length of 460 mu m. The influence of diced edges results in a volumetric efficiency of 59%, an active volume of 1.2 mm(2) of total 2.1 mm(2).
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